Peng et al. proposed[1]} a focus on hydrogen clathrate structures as a route to RTS. Results for \({\cal M}\) H\(_n\)  structures with \({\cal M}\) =Y and La, for \(n\) =6, 9, 10, and \({\cal M}\) =Sc for \(n\) =6 and 9 were presented.
The results pertinent for this discussion are for two yttrium hydrides. YH\(_9\)  at 150 GPa has the largest calculated coupling: \(\lambda \)\(\approx \) 4, T\(_c\)\(\approx \) 250K.
However, YH\(_9\)  at 400 GPa has an even larger predicted T\(_c\) : T\(_c\)\(\approx \) 290-300K with a smaller \(\lambda \)\(\approx \) 2.3. Considering the differences in structures and in optimum pressures, even their substantial amount of data only begins to provide guidelines for just what factors are most important in promoting RTS.
